Cryptography and .NET Framework - Part 3 (Public Key Encryption)

Introduction

In the Part 2 of this series we learnt to use Secret Key Encryption techniques using Triple-DES algorithm. More often to transfer data encrypted via secret key encryption public key encryption is used. This puts one extra layer of security over the data being transferred. From Part 1 we know that public key encryption consists of two keys - public key and private key. Data encrypted by public key can be decrypted only by the corresponding private key and vice a versa. One of the most popular algorithm for encrypting and decrypting data using this technique is RSA. The acronym RSA stands for Rivest, Shamir, and Adelman who are the inventors of the technique. The .NET framework provides a class called RSACryptoServiceProvider that encapsulates this algorithm. In this article we are going to learn how to use this class to secure your data.

Developing a class for encryption and decryption

Many developers don't want to go into the internals of Cryptography. They simply need a quick and easy way to secure their data. So we are going to develop such reusable class that will do the job of encrypting and decrypting for us.

We will create a class called PublicKeySecurityHelper which will have two methods - Encrypt and Decrypt. In addition we will also create a helper class called MyRSAInfo. This class will simply store certain pieces of data (such as public key and private key).

Here, is the complete code of the class.

Imports System.Security.Cryptography
Imports System.IO
Imports System.Text

Public Class PublicKeySecurityHelper
    Public Function Encrypt(ByVal strData As String) As MyRSAInfo
        Dim myrsa As New MyRSAInfo
        Dim p As CspParameters = New CspParameters
        p.Flags = CspProviderFlags.UseMachineKeyStore
        myrsa.Parameters = p
        Dim rsa As RSACryptoServiceProvider = 
	New RSACryptoServiceProvider(p)
        Dim data() As Byte = 
	rsa.Encrypt(Encoding.Unicode.GetBytes(strData), False)
        myrsa.PublicKey = rsa.ToXmlString(False)
        myrsa.PrivateKey = rsa.ToXmlString(True)
        myrsa.Data = data
        Return myrsa
    End Function

    Public Function Decrypt(ByVal myrsa As MyRSAInfo) As Byte()
        Dim rsa As RSACryptoServiceProvider = 
	New RSACryptoServiceProvider(myrsa.Parameters)
        rsa.FromXmlString(myrsa.PublicKey)
        Dim data() As Byte = rsa.Decrypt(myrsa.Data, False)
        Return data
    End Function
End Class

Public Class MyRSAInfo
    Public PublicKey As String
    Public PrivateKey As String
    Public Parameters As CspParameters
    Public Data() As Byte
End Class

Let's dissect the code step by step:

Encrypting data

• First we import the required namespaces. Especially System.Security.Cryptography is important one because it contains our core class RSACryptoServiceProvider
• We create a method called Encrypt() that accepts the string to be encrypted and returns an instance of a class called MyRSAInfo
• MyRSAInfo is our custom class defined at the bottom of the code. It consists of four public members - PublicKey, PrivateKey, Parameters and Data
• The PublicKey and PrivateKey members store the generated public key and private key respectively.
• The Parameters variable is of type CspParameters. This is used to automatically generate public and private keys and reuse them later on.
• The Data is an array of bytes and stores the encrypted version of the data
• Inside the Encrypt() method we create an instance of CspParameters class and set its Flag property to CspProviderFlags.UseMachineKeyStore. This enumerated value specifies from where the key information should be picked up i.e. from default key container or from machine level key store.
• Then we create new instance of RSACryptoServiceProvider class passing the CspParameters instance.
• We then call Encrypt() method of RSACryptoServiceProvider class and pass data to be encrypted. Since this parameter is byte array we convert our string into byte array using GetBytes() method. The second parameter of the method indicates whether to use OAEP padding (true) or PKCS#1 v1.5 padding (false). The former can be used only on Windows XP machines and hence we pass False. The Encrypt() method of RSACryptoServiceProvider class returns a byte array that contains encrypted version of the data.
• Finally, we fill all the members of MyRSAInfo class and return to the caller. Note how we call ToXmlString() method first passing False and then passing True to get public and private keys respectively.

Decrypting data

• In order to decrypt the data we create a method called Decrypt() that accepts an instance of MyRSAInfo class. This instance must be the one returned by the Encrypt() method explained earlier.
• Inside Decrypt() method we create an instance of RSACryptoServiceProvider class again passing the same CspParameters.
• We then call FromXmlString() method of the RSACryptoServiceProvider class and pass the public key generated before.
• Finally, we call Decrypt() method of RSACryptoServiceProvider class and pass the encrypted data. The second parameter of Decrypt method has the same significance as that of the corresponding parameter of Encrypt() method

Download

Complete source code along with a sample usage is available for download with this article (see top).

Summary

Public key encryption is a secure way to transfer data over networks. The fact that the private key is not sent in unsafe manner makes it more secure and robust. This technique is used in Secure Socket Layer (SSL) or HTTPS based web sites. The .NET framework class RSACryptoServiceProvider allows you to generate public and private keys, encrypt and decrypt data. In the next article we will learn about digital signatures. Stay tuned!